Electromagnetic interference (EMI) is a disturbance that interrupts, obstructs, or otherwise degrades or limits the effective performance of a circuit. Electromagnetic compatibility (EMC) is intended to ensure that circuits will not interfere with or prevent each other's operation because of EMI absorption.
A driving circuit for a light-emitting diode (LED) light source usually includes a converter for receiving an alternating-current input voltage from the grid and for generating a direct-current output voltage to drive the LED source. The converter turns a switch on and off according to a pulse-width-modulation (PWM) signal, such that the LED source is powered and the dimming controlled. However, because of the on and off operation of the switch, the current through the LED source is periodic and non-sinusoidal, composed of a sinusoidal current of a fundamental frequency and multiple sinusoidal currents of harmonic frequencies in a spectrum analysis. A harmonic frequency is an integral multiple of a fundamental frequency, for example, the secondary harmonic frequency of a fundamental frequency 50 Hz is 100 Hz, and the third harmonic frequency is 150 Hz. Thus, the current flowing through the LED source may further comprise a secondary harmonic, a third harmonic, and even more upper-harmonics. By either electromagnetic induction or radiation, the harmonic currents will enter other light-current systems (such as video systems or audio systems) in the same grid and interrupt their operations. Therefore, a conventional driving circuit for the LED light source has relatively poor EMC.
Switching frequency modulation is a conventional method to reduce EMI (see “Reduction of Power Supply EMI Emission by Switching Frequency Modulation”, IEEE Transactions on Power Electronics, Vol. 9, No. 1, January 1994, by Feng Lin, Member, IEEE, and Dan Y. Chen, Senior Member, IEEE). The converter creates side-bands by modulating the switching frequency, and thus the radiation characteristics of the harmonic currents are converted from a narrow-band noise to a broad-band noise. For example, by modulating the switching frequency in a preset range regularly or randomly, the noise energy is distributed into smaller pieces scattered around side-band frequencies, such that a peak current at the harmonic frequency is attenuated effectively. Thus, EMI is reduced. However, the LED current changes as the switching frequency changes, which will cause the LED light source to flicker. Therefore, the LED light source has poor current stability.